In a technical environment such as IBM's ESCON I/O interface, (see for example IBM publication SA22-7202-02, "ESA/390 ESCON I/O Interface"), it is required that a logical path be established between a channel and a control unit before communication for I/O from the processor can proceed. Control units have limitations on the maximum number of logical paths (and physical paths) they can support. This limits the number of channels (and thereby processors and systems) that can access the control units even though the dynamic switch located between the channel and the control unit may have a lesser limitation on logical paths. Prior art allows the ability to dynamically disconnect and connect channels and control units via serial I/O links and a dynamic switch. This facility is described in U.S. Pat. No. 5,107,489, "Switch and its Protocol for Making Dynamic Connections" issued Apr. 21, 1992, by Paul J. Brown, et al., assigned to the assignee of the present invention, and incorporated herein by reference. In prior art the capability exists to share physical paths (via logical paths), but the capability of this sharing is limited by the capacity of the control unit. The number of concurrent connections is limited by the maximum number of logical paths the control unit can support. In the prior art once the maximum number of logical paths a control unit can support are connected, any further requests for connection (for example from other processors) are rejected. This rejection results in an inability to initiate any work in the rejected processor that would utilize I/O devices attached to the rejecting control unit. This limitation of the number of logical paths proves to be a significant connectivity constraint, especially since the ESCON topology allows all channels (processors) connecting to a dynamic switch to have access to all control units (and I/O devices) connected to that dynamic switch. This is referred to as any to any connectivity.
For I/O devices that support dynamic pathing (which includes all DASD devices attached through the ESCON I/O interface), in IBM's MVS/ESA operating system implementation, if the last path to the device becomes not operational (due to errors, configuration failures, etc) the device is isolated, "boxed", by the I/O supervisor program. This boxing forces the device to become off-line and forces all work currently allocated to the device to have outstanding, or future, I/O fail. This usually causes the work to fail and be terminated.